This disclosure is directed to flow measurement within a borehole and in particular within a cased borehole where there is a suspicion that the casing has a leak creating flow on the exterior.
It is important to measure flow velocity in a well and in particular in a cased well where there is suspicion of leaking through the casing and drainage into adjacent formations. The present method also contemplates measurement of flow to determine the direction of flow, the velocity, all within the casing, and also flow on the exterior. When a casing is first installed in a well, it is normally cemented in place so that casing integrity is assured. Over a period of time, leaks may be formed through the casing. Indeed, it is possible for a well to leak through the casing, through the cement, and along the well (upwardly or downwardly) to a nearby formation which may accept the fluid flow. The present method is a procedure which first tests the simple situation of flow in the well (assuming no leaks), and determines the direction of flow and actual velocity. In a more sophisticated approach, the present method also measures flow velocity in the well, the leakage fluid flow velocity on the exterior of the casing and cement and the direction of each flow. Remedial treatment steps can be undertaken to plug the leak and thereby prevent leakage to the exterior. This avoids potential pollution flow from a producing well along an unintended pathway into artesian formations which may poison formations intended to be isolated from well production fluids. This procedure protects isolated formations from the invasion of production fluids. The production fluid may include a substantial portion of salt water. Where the well penetrates an artesian sand having only pure water, it is highly detrimental to that formation if salt water leaks into it.
Because of the pressure differentials that may prevail in various formations, leaks in well bore casing can be detrimental to hydrocarbon reservoirs. Water channeling through casing and cement leaks are detrimental to optimum production.
The present method contemplates the use of first and second tracer fluids which are injected into the well. They are radioactive tracer isotopes emitting gamma radiation. While in one procedure they can be identical isotopes, a greater range of testing can be accomplished through the use of different radioactive isotopes emitting gamma radiation of distinctly different energies. It is desirable that the isotopes be different in the sense that passage of the two different isotopes can be distinguished by suitable gamma ray detectors. Two spaced detectors are included in a sonde which carries two separate radioactive isotope injectors located at opposite ends of the sonde. If the isotopes are different, the two detectors can separately identify passage of the two isotopes as for instance by spectrum stripping, spectrum fitting, or by other known and suitable procedures. One suitable spectroscopic technique is set forth in U.S. Pat. No. 4,035,640.